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One of the most obvious differences between modern life and life in the past can be found in the level of exercise. For the majority of people living in developed countries today, heavy physical exercise does not occur as a part of ordinary daily life, but must be deliberately sought out. Compare this to most of human history, in which heavy daily exercise was a requirement for survival. Even among the upper classes in 19th century Europe—to judge by a scene in Charles Dickens
Pickwick Papers
—going for a 10 to 20 mile walk by way of recreation was not be out of the ordinary course of events.

The human body was designed to use its physical capacities. However, for many of us, life has become a sedentary affair, moving from couch via car to office cubicle. While decreasing strenuous exercise does have some benefits, such as reducing injuries, it also presents major drawbacks. Inadequate exercise is undoubtedly a major contributor to the current epidemic of obesity, which in turn leads to diabetes, heart disease, and osteoarthritis.

Conversely, increasing one's level of exercise provides a wide variety of benefits. Besides enhancing strength and endurance and improving physical attractiveness, exercise is thought to enhance overall health as well as reduce symptoms in a number of specific ailments.
However, while the many benefits of exercise appear self-evident, they can be quite difficult to
prove
in a scientific sense. The primary problem comes down to this: it is difficult, if not impossible, to design a double-blind study of exercise.

In a double-blind, placebo-controlled study, neither patients nor researchers know who is receiving a real treatment and who is receiving a placebo. The centrality of such studies is discussed in detail in
Why Does This Database Rely on Double-blind Studies?
We will discuss the subject here only in brief.

Consider the following scenario: A study (technically, an
observational
or epidemiological study) may note that people in a given population who exercise more develop heart disease at a lower rate than those who exercise less. From this, it is tempting to conclude causality: that exercise
reduces
heart disease risk. But such a conclusion might not be correct.

Observational studies only show association, not cause and effect. Studies of the type described above had long shown that women who used hormone replacement therapy (HRT) were less likely to develop heart disease. Furthermore, use of HRT was known to improve cholesterol profile.
It seemed like a "slam-dunk" case. However, to researchers' surprise, when a giant double-blind study compared hormone replacement therapy against a placebo, the results showed that use of HRT actually
increased
heart disease risk.

It is now hypothesized that this apparent contradiction may be due to the fact that women who use HRT are generally of higher socioeconomic status than women who do not use HRT, and that it is this socioeconomic status, and not the HRT, that was responsible for the apparent benefits seen. Whatever the reason, it is now clear that HRT does not prevent heart disease, and that the conclusions drawn from observational studies were exactly backwards. Based on this, one must at least consider the possibility that people who engage in more exercise have other qualities that protect them from heart disease, and that it is these qualities, and not the exercise, that protects them.
The problem here is that while it is possible to give a placebo that convincingly resembles HRT, it is difficult to conceive of a placebo form of exercise that patients and researchers wouldn't immediately identify as different from real exercise.

Besides observational studies, other forms of scientific research involving exercise remain similarly inadequate. For example, consider the numerous studies that have been taken as proving that exercise is helpful for
depression
. In these studies, people who are made to exercise improve to a greater extent than those who are not interfered with. However, this finding does not prove that exercise
per se
aids depression. It might be, for example, that simply being enrolled in a study and motivated to do anything at all might aid depression. (This suspicion is given further weight by findings that improvement in depression is not at all related to the intensity of the exercise done—if it were the exercise itself, one would think that more intense exercise would provide greater benefits.)

Double-blind, placebo-controlled studies eliminate all of these potential confounding factors, as well as many others. However, as noted above, it is not feasible to design a double-blind study in which people are unaware (“blind” to the fact ) that they are exercising. Therefore, all results regarding the potential benefits of exercise must be taken with a grain of salt.

What Is the Scientific Evidence for Exercise?

Keeping the above discussion in mind, the benefits of exercise with the most solid scientific foundation
include:
1-4

Regarding blood pressure, aerobic exercise has the best supporting evidence, but resistance exercise (weight training) has also shown promise.
5-9
One interesting study found that four 10-minute "snacks" of aerobic exercise per day were as effective at lowering blood pressure as 40 minutes of continuous exercise.
9

It is widely believed that exercise improves immune function. However, there is no meaningful supporting evidence for this belief. Very high intensity exercise (such as marathon running) is known to temporarily weaken the immune system, increasing likelihood of respiratory infection. This is discussed in the article
Sports and Fitness Support: Enhancing Recovery
.

Evidence conflicts on whether exercise is helpful for reducing
menopausal symptoms32,33
However, it is known that heavy exercise causes increased calcium loss through sweat, and the the body does not compensate for this by reducing calcium loss in the urine.
35
The result can be a net calcium loss great enough so that it presents health concerns for
menopausal
women. One study found that use of an inexpensive calcium supplement (calcium carbonate), taken at a dose of 400 mg twice daily, is sufficient to offset this loss.
35

Aerobic exercise however, may be beneficial for sedentary women. In a randomized trial of 176 women who had their last menstrual period within 3-36 months, aerobic exercise was associated with a decrease in menopausal symptoms including night sweats, irritability, depression, mood swings, headache, and urinary problems. The trial compared unsupervised aerobic exercise for 50 minutes, 4 times per week to twice monthly health lectures.
37

Revision Information

This content is reviewed regularly and is updated when new and relevant evidence is made available. This information is neither intended nor implied to be a substitute for professional medical advice. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with questions regarding a medical condition.

Kujala UM. Evidence for exercise therapy in the treatment of chronic disease based on at least three randomized controlled trials--summary of published systematic reviews.
Scand J Med Sci Sports.
2004;14:339-45.

Lawlor DA, Hopker SW. The effectiveness of exercise as an intervention in the management of depression: systematic review and meta-regression analysis of randomised controlled trials.
BMJ.
2001;322:763-767.